[PATCH v2 4/4] arm64: Remove unneeded rcu_read_lock from debug handlers

From: Masami Hiramatsu <mhiramat@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>
Cc: mhiramat@kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	Dan Rue <dan.rue@linaro.org>, Matt Hart <matthew.hart@linaro.org>,
	Anders Roxell <anders.roxell@linaro.org>,
	Daniel Diaz <daniel.diaz@linaro.org>,
	"Paul E . McKenney" <paulmck@linux.ibm.com>
Subject: [PATCH v2 4/4] arm64: Remove unneeded rcu_read_lock from debug handlers
Date: Mon, 22 Jul 2019 16:49:11 +0900	[thread overview]
Message-ID: <156378175027.12011.5591853683946780785.stgit@devnote2> (raw)
In-Reply-To: <156378170297.12011.17385386326930403235.stgit@devnote2>

Remove rcu_read_lock()/rcu_read_unlock() from debug exception
handlers since we are sure those are not preemptible and
interrupts are off.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.ibm.com>
---
 arch/arm64/kernel/debug-monitors.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index f8719bd30850..48222a4760c2 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -207,16 +207,16 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
 
 	list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
 
-	rcu_read_lock();
-
+	/*
+	 * Since single-step exception disables interrupt, this function is
+	 * entirely not preemptible, and we can use rcu list safely here.
+	 */
 	list_for_each_entry_rcu(hook, list, node)	{
 		retval = hook->fn(regs, esr);
 		if (retval == DBG_HOOK_HANDLED)
 			break;
 	}
 
-	rcu_read_unlock();
-
 	return retval;
 }
 NOKPROBE_SYMBOL(call_step_hook);
@@ -305,14 +305,16 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
 
 	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
 
-	rcu_read_lock();
+	/*
+	 * Since brk exception disables interrupt, this function is
+	 * entirely not preemptible, and we can use rcu list safely here.
+	 */
 	list_for_each_entry_rcu(hook, list, node) {
 		unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
 
 		if ((comment & ~hook->mask) == hook->imm)
 			fn = hook->fn;
 	}
-	rcu_read_unlock();
 
 	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
 }

